The present invention lies in the field of ore benefication using froth flotation processes. It is particularly directed to the use of a bacterial cellulose as a readily floatable silicate mineral depressant.
A high percentage of the metal ores mined today are of relatively low quality; i.e., the content of the metal-bearing mineral in the ore is very low in relation to the nonmetallic matrix minerals. As one example, it has been calculated that the copper content of a typical city garbage landfill is appreciably higher than that of most of the ores currently being mined. The first significant process step after mining is that of ore benefication. This is a primary separation of the desired metal ore mineral from the great bulk of the gangue in which it naturally occurs. In some parts of the world, especially for high value precious metal ores, an initial hand separation of ore is still made. However, in most locations high labor costs dictate the use of other methods. For most nonferrous minerals, and even in some instances where iron ores are being processed, froth flotation is the preferred method of ore benefication.
In a froth flotation process the ore is first finely ground to release the desired mineral from the gangue in which it is embedded and dispersed. Various conditioning agents may or may not be added during grinding. The ground ore is then dispersed as a high consistency pulp or slurry in water. Various chemical agents are added so that the minerals of value are either selectively wetted or made hydrophobic relative to the other mineral components. After a period of conditioning during which this surface modification of the particles takes place, air in the form of fine bubbles is introduced into the flotation cell. Those particles that are the most hydrophobic will become attached to an air bubble and be carried to the surface where they are held in a froth. The froth is then skimmed to recover the contained material.
Normally it is desirable to depress the waste material into the tailings from the flotation cell with the desired minerals being carried into the froth. However, occasionally the nature of the ore will dictate the reverse procedure. The usual flotation is a continuous process that involves several well defined stages and may include regrinding one or both of the accepted and tailings components. The most usual procedure is to further concentrate the component recovered in the froth from an initial "rougher" stage in one or more "cleaner" stages to further increase the ratio of minerals to matrix rock components. Rougher tailings can be further processed in a "scavenger" flotation if the value of the residual minerals is sufficiently high. The particular flotation process, viewed in its entirety, will depend very much on the mineralogy and economic value of the ore being processed and will be specifically tailored to that situation.
Ore beneficiation processes are usually located very near the mine site to minimize shipping and disposal costs of large amounts of valueless tailings. Since no flotation process is 100% efficient, there is always some loss of the desired mineral in the tailings and this loss occurs at every flotation stage. If the concentrate is to be shipped to a refinery a considerable distance from the mine site it may be more economical to accept a somewhat lower mineral recovery; i.e., higher process losses, in order to make the concentrate grade as high as possible. The savings in shipping costs may well offset the incremental loss of the desires mineral. On the other hand, if the refinery is nearby, a lower grade product may be entirely acceptable in order to maximize recovery. Economic considerations such as these must enter into the design of the flotation unit.
It is very common for an ore to contain economic amounts of several minerals. An example would be copper ores with significant amounts of other useful metals such as lead, zinc, cadmium and smaller quantities of precious metals such as silver and gold. In this case, secondary or tertiary flotation steps may be done to further separate the individual mineral components. An example might be separation of galena, a lead sulfide, from sphalerite, a zinc sulfide. Different chemicals will be required here to float the lead and zinc sulfide separately. An example is described in the paper of Bakinov et al., New Methods of Sulfide Concentrate Upgrading, VII International Minerals Processing Congress, Technical Papers, Sept. 20-24, 1964, Vol. 1, pp 227-23 et seq, Gordon and Breach Science Publishers, Inc. New York. Another paper pertinent to this type separation is Jin et al., Flotation of Sphalerite from Galena with Sodium Carboxymethyl Cellulose as a Depressant, Preprint 87-23, Society of Mining Engineers, Annual Meeting, Feb. 24-27, 1987, Denver, Colo. Reference might also be made to Shaw, U.S. Pat. No. 4,268,380 and Ramadorai and Shaw, U.S. Pat. No. 4,329,223 for general background information on multistage separations using flotation.
Flotation chemicals can be generally classified as collectors, depressants, frothers, and modifiers. Collectors are materials that selectively render hydrophobic the surface of particles to be floated and enable them to become attached to the air bubbles rising to the surface of the cell rather than remaining with the gangue or tailings. Typical collector materials are oleic acid; various xanthate salts such as alkali metal salts of propyl, butyl or amyl xanthate; salts of thiocarboxylic acids; mercaptans; and dialkyldithiophosphates. Choice of the collector will depend very much on the nature of the minerals to be recovered in the froth; e.g., sulfide minerals will usually require different collectors than oxide or carbonate minerals.
Depressants, on the other hand, are materials that selectively modify particle surfaces so that they become hydrophilic; i.e., they inhibit adsorption of collectors and reduce the tendency of the mineral to become attached to the rising air bubbles. These are often natural or synthetic gums or polysaccharides such as guar, arabinogalactans, starch, dextrins, hemicelluloses, sodium carboxymethylcellulose, or sodium cellulose sulfate. Other materials occasionally used are a cuprammonium complex of cellulose, Noke's Reageant (a P.sub.2 S.sub.5 -NaOH reaction product), thiocarboxylic acids, and inorganic materials such as sodium sulfide, sodium silicate, and sodium cyanide.
Frothers are usually water insoluble materials that promote foaming by reducing the surface tension of the water. Among them are monohydric long chain alcohols, various resinates, cresylic acid, terpineol, pine oil and methylisobutyl carbinol.
Modifiers or activators include a wide variety of chemicals having various functions. One such function is to modify the surface of a mineral so that a collector either does or does not adsorb on it. These include materials having such diverse functions as pH adjustment, removal of a collector from mineral surfaces between different flotation stages, etc. Activated carbon would be an example of a material intended for the last mentioned use as is described in the aforementioned patents to Shaw and Ramadorai et al.
The lists of chemicals given above should be regarded as exemplary only and are not intended to be all inclusive.
Among the particularly troublesome minerals to depress into the gangue are those generally classified as readily floatable silicate (RFS) minerals. These are often referred to as talcose minerals and include minerals having a plate-like structure such as talc, phlogopite, and serpentine. Fibrous asbestos group materials such as actinolite and tremolite present similar problems. Ores that present this difficulty are generally referred to as high talc or high RFS ores.
The physical chemistry of flotation processes is extremely complex and is not highly predictable for new ore sources. As one example, Rhodes examines the effect of variables in carboxymethyl cellulose on nickel recovery from an Australian talc containing ore. Significant differences in depressant performance are found depending on the degree of substitution, the degree of polymerization (viscosity) and the temperature history of solutions of the carboxymethyl cellulose used in the process (Rhodes, M. K., in Mineral Processing, Proceedings, Part A, Thirteenth International Mineral Processing Congress, Warsaw, June 4-9, 1979, pp 346-367, Elsevier Scientific Publishing Company, New York).
South African patent application No. 882,394 describes the use of hemicellulose obtained from various sources as a talc depressant for ore flotation. This document gives a good basic background description of ore flotation processes.
Carboxymethylcellulose has been known as a readily floatable silicate mineral depressant since the 1940s. Despite its availability in many chemical variations of substitution and molecular weight, and many years of experience with its use and the use of other depressant materials, the mining industry is still looking for new materials that will improve flotation efficiency. Quite unexpectedly the bacterial cellulose product of the present invention appears to serve such a need.